1. Field of the Invention
The present invention relates to a radio transmission apparatus, and more particularly to a radio transmission apparatus transmitting a signal using a microwave signal from a home or a small business, in two-way communication via a broadcasting satellite or a communication satellite between a broadcasting station and the home or the small business.
2. Description of the Background Art
In recent years, a variety of radio communication systems using a microwave signal have been developed by utilizing a broadcasting satellite or a communication satellite, and a radio communication market has dramatically grown accordingly. Demands for two-way communication in the radio communication market have been increasing with the development of the internet, the introduction of BS digital broadcasting, or the like.
Conventionally, in the two-way radio communication targeting a home or the like, only signal transmission in downstream from a broadcasting station to a home is carried out via radio communication while signal transmission in upstream from a home to a broadcasting station mainly utilizes a telephone line.
Although the telephone line used in the upstream allows an efficient use of the existing infrastructure, it has a slow transmission speed and thus is not suitable for data communication with a large volume of information such as animation data or the like, which may be obstruct the upcoming, full-fledged two-way communication age. An attempt has then been made to use the satellite communication also in the upstream from a home to a broadcasting station.
FIG. 8 is a schematic diagram conceptually illustrating a two-way communication system between a broadcasting station and each home via an artificial satellite. With reference to FIG. 8, a two-way communication system 200 includes a broadcasting station 201, homes 202, 203, a satellite 204, and parabolic antennas 205-207.
Broadcasting station 201 is a base station which converts information such as image, music or the like into a microwave signal for transmission from parabolic antenna 205 to satellite 204. Broadcasting station 201 also receives information transmitted from homes 202, 203 via satellite 204.
Homes 202, 203 are users that receive and make use of the information such as image, music or the like sent from broadcasting station 201 via satellite 204. Homes 202, 203 receive the microwave signal carrying information such as image, music or the like from satellite 204 using parabolic antennas 206, 207 and convert the microwave signal into information such as image, music or the like using an LNB (Low Noise Blockdown converter) in a manner similar to the conventional satellite broadcasting receiving system. Homes 202, 203 also convert the information that the users want to transmit into a microwave signal using a radio transmission apparatus described later, and transmit that signal from parabolic antennas 206, 207 to satellite 204. The microwave for use in the two-way communication system 200 has a frequency of a 14-14.5 GHz band.
Satellite 204 is an artificial satellite that relays microwave signals between broadcasting station 201 and homes 202, 203.
Parabolic antennas 205-207 are antennas placed at broadcasting station 201 and homes 202, 203 on the earth for receiving/transmitting microwave signals from/to satellite 204.
In two-way communication system 200, broadcasting station 201 converts information such as image, music or the like into a microwave signal for transmission from parabolic antenna 205 to satellite 204. Homes 202, 203 receive the microwave signal transmitted by broadcasting station 201 from satellite 204 with parabolic antennas 206, 207 and convert the microwave signal into information such as image, music or the like using LNB for acquisition.
On the other hand, when information is sent from homes 202, 203 to broadcasting station 201, homes 202, 203 convert the information to an intermediate frequency signal at an In Door Unit (not shown), convert the intermediate frequency signal to a microwave signal using a radio transmission apparatus (described below) installed at homes 202, 203, and transmit that microwave signal from parabolic antennas 206, 207 to satellite 204. Broadcasting station 201 then receives the microwave signal via satellite 204 using parabolic antenna 205 and acquires the information sent from homes 202, 203.
FIG. 9 is a schematic block diagram functionally illustrating a conventional radio transmission apparatus used at homes 202, 203 in two-way communication system 200. Referring to FIG. 9, a radio transmission apparatus 300 includes an input terminal 301, an input processing circuit 302, a frequency conversion circuit 303, a transmission power amplifying circuit 304, and a power control circuit 305. Input processing circuit 302 includes a capacitor 3021 and a coil 3022. Frequency conversion circuit 303 includes a mixer circuit 3031 and a local oscillation circuit 3032. Transmission power amplifying circuit 304 includes a power amplifier 3041 and a microwave amplifier 3042.
Input terminal 301 is a terminal inputting an intermediate frequency signal output from an In Door Unit (also abbreviated as IDU, not shown in the figure) through a coaxial cable. The intermediate frequency signal is a signal produced by modulating the information that the user wants to transmit to broadcasting station 201 into a signal having a frequency band of about 950-1450 MHz, and this intermediate frequency signal is superposed on a direct current (DC) voltage of approximately 13-26 V for input into input terminal 301.
Input processing circuit 302 separates an input voltage input to input terminal 301 into a DC voltage used as the power for radio transmission apparatus 300 and an intermediate frequency signal as a transmission signal.
Capacitor 3021 prevents the DC voltage input from input terminal 301 from being input to frequency conversion circuit 303, and separately acquires only the intermediate frequency signal from the input voltage input to input terminal 301.
Coil 3022 is provided to remove the intermediate frequency signal from the input voltage input from input terminal 301 to obtain the DC voltage used as the operating power for radio transmission apparatus 300.
Frequency conversion circuit 303 converts the intermediate frequency signal into a microwave signal having a higher frequency suitable for radio transmission.
Mixer circuit 3031 mixes the intermediate frequency signal with a local oscillation signal produced by local oscillation circuit 3032 to convert the intermediate frequency signal into the microwave signal.
Local oscillation circuit 3032 is a circuit that produces a local oscillation signal used in converting the intermediate frequency signal into the microwave signal at mixer circuit 3031.
Transmission power amplifying circuit 304 amplifies the microwave signal converted at frequency conversion circuit 303 for output to a parabolic antenna.
Power amplifier 3041 and microwave amplifier 3042 both amplify the microwave signal. The gain of power amplifier 3041, however, is limited. Microwave amplifier 3042 is thus provided prior to power amplifier 3041 in order to ensure the input level of power amplifier 3041 at a high level.
Power control circuit 305 inputs the DC voltage resulting after the intermediate frequency signal has been removed by coil 3022, and supplies the operating power to frequency conversion circuit 303 and transmission power amplifying circuit 304.
In radio transmission apparatus 300, the intermediate frequency signal superposed on the DC voltage is input from input terminal 301 to input processing circuit 302. Capacitor 3021 extracts the superposed intermediate frequency signal component from the input voltage as being input and outputs the intermediate frequency signal to frequency conversion circuit 303. Frequency conversion circuit 303 mixes the intermediate frequency signal with the local oscillation signal produced by local oscillation circuit 3032 for conversion into a microwave signal, and outputs the microwave signal to transmission power amplifying circuit 304. Transmission power amplifying circuit 304 uses microwave amplifier 3042 and power amplifier 3041 to amplify the microwave signal for output. The microwave signal output from transmission power amplifying circuit 304 is then transmitted from the parabolic antenna to satellite 204.
On the other hand, coil 3022 removes the intermediate frequency signal from the input voltage including the superposed intermediate frequency signal and outputs the DC voltage to power control circuit 305. Power control circuit 305 then converts the DC voltage resulting after the signal component has been removed to a prescribed voltage, and supplies the power to frequency conversion circuit 303 and transmission power amplifying circuit 304.
In the conventional radio transmission apparatus 300, since the DC voltage is continuously input to input terminal 301 even when the output of the intermediate frequency signal from In Door Unit is intentionally stopped, the power is supplied from power control circuit 305 to frequency conversion circuit 303 and transmission power amplifying circuit 304. As a result, even if the intermediate frequency signal is not transmitted, frequency conversion circuit 303 and transmission power amplifying circuit 304 operate, thereby causing an undesirable power consumption.
Furthermore, since transmission power amplifying circuit 304 operates even if the output of the intermediate frequency signal is intentionally stopped, a noise in a transmission frequency band is amplified and the high level noise is transmitted to the artificial satellite, thereby causing a communication error.